That's a nice problem to have.
We absolutely loved Stockholm and were only there for a port day, so I would definitely recommend that you stay there for 2 days. We were able to do the Frommers walking tour of the Gamla Stan (you'll be able to see the royal palace, changing of the guard, etc. there) and then took a ferry to the Djurgården peninsula where we saw the Vasa Museum and Skansen. I would recommend that you spend a whole day in the Gamla Stan/royal palace area with an added visit to the Kaknästornet television tower for a panoramic view of the city and its archipelago and a whole day on the peninsula. Skansen was absolutely charming (http://www.skansen.se/eng/hgtindex.htm
), and we enjoyed the Vasa Museum http://www.vasamuseet.se/
) more than the Viking Ship Museum in Norway, although you really should not miss that.
I would spend the other two days in London. You didn't mention whether or not you have been to London on previous trips, but there's always so much to do there. When we were there last month, my husband took the train to Portsmouth and spent the whole day exploring the Royal Naval Museum and all the ships there while I did a return visit to Kew Gardens and the National Gallery. (The exhibit at the Hermitage Rooms is not very good this spring/summer.) Other memorable London experiences are the side trip to Windsor (the changing of the guard is better there than it is in London, it's a lovely town, and the castle is fabulous), Imperial War Museum, Apsley House, London Shopping Tour (see the Born to Shop London book and knock yourself out!), British Museum, Victoria & Albert Museum, Imperial War Museum, Cabinet War Rooms, the Courtauld Collection, British Museum, Hampton Court Palace, Houses of Parliament, St. Paul's Cathedral, the Tower of London, and the Tower Bridge (there's a museum inside, and you can walk along the walkways up high for good views of the Thames), Westminster Abbey, etc. etc. etc. Also, for evening activities there are the theaters, and we have a delightful evening riding the London Eye followed by a Thames River Cruise last May. I also recommend a side trip to Bath and the Cotswolds if you have seen London (although, as noted above, that's really hard to do). Please feel free to e-mail me if you need any additional information that I can provide on any of these attractions.
Hope you have a great time.